1. Field of the Invention
The present invention generally relates to apparatus for detecting digital data and, more particularly, is directed to an apparatus for detecting digital data for use in a video tape recorder (VTR) which uses Class IV or other partial response signalling in high density digital tape recording and reproducing.
2. Description of the Prior Art
So-called digital video tape recorders have been developed to record a video signal in the form of a digital signal on a magnetic tape and, with such digital VTRs, the deterioration of image quality in the dubbing mode can be suppressed to a minimum.
Incidentally, when a signal is recorded on and/or reproduced from a magnetic tape, the electromagnetic transducing system used therefor, such as, a magnetic head or the like, has a differentiation characteristic so that the C/N (carrier-to-noise) ratio is deteriorated at the lower frequency side. The C/N ratio will be similarly deteriorated due to a magnetization characteristic of the magnetic tape itself, shown in FIG. 1, if the frequency is increased.
Accordingly, when using a magnetic recording and/or reproducing system for a digitized video signal (hereinafter, referred to as a digital video signal), the frequency band in which a satisfactory C/N ratio can be obtained is relatively narrow. For this reason, when a digital video signal is to be detected, a detection system is employed having a signal spectrum concentrated near a region in which the C/N ratio is maximized so as to effectively avoid deterioration of the C/N ratio of the reproduced signal, and thereby ensure that the digital video signal will be recorded and/or reproduced efficiently. In connection with the foregoing, it has been proposed to utilize a class IV partial response scheme in the reproduction and detection of a digital video signal, for example, as disclosed in U.S. Pat. No. 4,504,872, issued Mar. 12, 1985, and in U.S. Pat. No. 4,984,099, issued Jan. 8, 1991, and which has a common assignee herewith.
More specifically, since the C/N ratio in a magnetic recording and/or reproducing system is deteriorated at the low and high frequencies, its frequency characteristic can be approximated by a frequency characteristic H(.omega.) of a class IV partial response (1-D.sup.2) scheme expressed by using a delay operator D as shown in FIG. 2.
Incidentally, the frequency .omega..sub.0 that is, the Nyquist frequency at which the response is minimized, has a relation to the delay time T imposed by the delay operator D as shown by the following equation: ##EQU1##
Accordingly, if the amount of delay imposed by the delay operator D is selected so that the signal spectrum is concentrated near the region in which the C/N ratio is maximized, then the digital video signal can be recorded and/or reproduced efficiently by effectively utilizing the frequency characteristic of the magnetic recording and/or reproducing system.
In other words, in the recording mode, a calculation process corresponding to the following expression (2) is sequentially performed on the digital video signal: ##EQU2## where MOD2 represents the remainder of 2.
Further, since the electromagnetic transducing system, for example, a magnetic head, has a differentiation characteristic, a reproduced signal from the magnetic head has a characteristic expressed as (1-D) with reference to the delay operator D and which is shown by the correspondingly labeled dashed line in FIG. 2.
Accordingly, in the playback or reproducing mode, a calculation process of (1+D) is performed on the reproduced signal, whereby the correction expressed by the following equation can be executed: EQU (1-D).(1+D)=1-D.sup.2 ( 3)
Owing to expression (2) and equation (3), the digital video signal can be reproduced with the transfer function of the recording and reproducing system being maintained at "1".
When the digital video signal is recorded and/or reproduced by making effective use of class IV partial response signalling, a digital video signal having a small bit error rate can be reproduced by the application of the Viterbi decoding technique, which indicates that, as shown in FIG. 2, the digital video signal can be efficiently detected by effectively utilizing a frequency characteristic nearly equal to the characteristic of FIG. 1 which represents the frequency characteristic of the magnetic recording and/or reproducing system.
For example, as disclosed in detail in "Analog Viterbi Decoding for High Speed Digital Satellite Channels", A. S. Acampora et al., IEEE Transactions on Communications, Vol. Com. 26, No. 10, October 1978, pages 1463-1470; and in "The Viterbi Algorithm", G. D. Forney, Jr., Proceedings of the IEEE, Vol. 61, No. 3, March 1973, pages 268-278, a Viterbi decoding circuit utilizes likelihood of correlation between data input successively thereto for detecting transit of the data and decodes the data on the basis of the detected result.
Accordingly, if the relationship (1-D.sup.2) of the reproduced signal relative to the signal used for recording (hereinafter referred to as the "recording signal") is utilized to decode the recording signal from the reproduced signal and then the digital video signal is decoded on the basis of the decoded data, the bit error rate of the decoded data can be reduced as compared with a standard decoding circuit which decodes data with reference to the signal level.
A known reproducing circuit of a digital VTR which incorporates class IV partial response and Viterbi decoding schemes will now be described with reference to FIG. 3 in which digital video data recorded as a binary signal in analog form on a video tape 1 is reproduced by a magnetic head 2. The reproduced signal is supplied through an amplifier 3 to an equalizer circuit 4, and the equalized reproduced signal output from the equalizer circuit 4 is supplied to a processing circuit 5. The processing circuit 5 performs on this reproduced signal the calculation (1+D) in accordance with the above-mentioned partial response scheme. A calculated output of the processing circuit 5 is supplied to an analog-to-digital (A/D) converter 6. A reproduced clock is generated from the reproduced signal by a phase-locked loop (PLL) circuit 7 to which the output of the amplifier 3 is supplied. This reproduced clock is supplied to the A/D converter 6 which detects digital data from the reproduced signal level on the basis of the reproduced clock. Detected digital data is supplied to a Viterbi decoder circuit 8, in which the data is decoded in accordance with the Viterbi decoding scheme to detect a digital video signal. The thus detected digital video signal is supplied through an output terminal 9 to a reproduced signal processor circuit (not shown) at a next or succeeding stage.
When the digital video signal is reproduced by the known circuit arrangement of FIG. 3, the transmission rate of the digital video data is very high so that the frequency of a clock necessary for processing reproduced data in each of the circuits must be selected to be higher than 300 MHz. The circuits which are operated at such high clock frequency require a calculation circuit of a special configuration, which cannot be readily provided in actual practice. Further, it is preferable that the equalizer circuit 4 and the processing circuit 5 be fabricated as digital circuits because digital equalizer circuits and digital processing circuits can provide excellent characteristics. However, it is very difficult to operate the equalizer circuit 4 and the processing circuit 5 at any clock frequency higher than 30 MHz if these circuits 4 and 5 are in digital form. Therefore, in the circuit according to the prior art shown on FIG. 3, the equalizer and processing circuits 4 and 5 precede the analog-to-digital converter 6 and are not fabricated as digital circuits.
Similarly, in U.S. Pat. No. 4,504,872 referred to above, the analog-to-digital conversion follows passage of the reproduced signal through a 1-D.sup.2 class IV response filter so that the latter is not fabricated as a digital circuit and, hence, does not realize the advantageous characteristics inherent in digital fabrication.